Perforated EMI gasket

ABSTRACT

A computer chassis includes a first metal portion and a second metal portion. A mating edge connection is provided between the first and second portions. A gasket is mounted in the edge connection. The gasket includes a compressible strip of electromagnetic interference (EMI) limiting material. A pattern of holes are formed in the strip to improve compressibility arid thus enhance EMI shielding.

BACKGROUND

[0001] The disclosures herein relates generally to computer systems andmore particularly to a perforated gasket for providing anelectromagnetic interference seal for a computer chassis enclosure.

[0002] There is a widespread problem of trying to close, or fill, gapsin chassis enclosures, especially removable-cover seams. The ability toclose these gaps is essential in order to pass the FCC's electromagneticinterference (EMI) requirement and well as electrostatic dischargesusceptibility.

[0003] Conductive foam gaskets have proven to be the most robust andcost effective solution to providing an EMI seal. However, traditionalfoam gaskets pose a number of problems.

[0004] The bigger/taller the gasket profile, or cross-section, thegreater it's range of compression. However, the problem is furthercomplicated by cover and chassis geometry. Firstly, a foam gasket isselected that, theoretically, gives the required range of compression,given the theoretical tolerances (and theoretical forces). But if thisgasket generates forces, which either deform the covers so subsequentgaps are created, or the net forces are too high for ergonomicrequirements, then a larger gasket is selected that generates less forcefor a given range of compression. Most often, both tolerances and actualforces contribute to the problem, invariably due to design changes andvariance in the parts throughout the product design/development cycles.However the chassis design must be revised to accommodate the largervolume gasket, if possible. Often the space is simply not available. Inthin rack servers this is the case because the residual height of thegasket after maximum allowable compression must be accommodated and thatspace is not available. When engineers initially “pad” their designswith excessive gasket volumes, the computer designs as a whole will besubsequently degraded from lost volume or other geometric/spaceconflicts. Whole-programs maybe abandoned or disabled due to thispractice. Therefore, any solution that incrementally reduces thecompressive forces relative to range of compression for a gasket helpstremendously.

[0005] Two other solutions are commonly used to solve the aboveproblems; custom spring fingers and wire mesh gaskets. Custom springfingers are far more expensive (if made from Beryllium Copper orPhosbronze) or not as resilient as foam core gaskets. Additionally,spring fingers are not as robust in terms of customer access as they caneasily hang up on passing objects, getting permanently deformed orbroken off. Wire mesh gaskets have an inherent problem with having to besealed at their ends to prevent unraveling. This causes the ends to betoo stiff, thereby countering the high compliance given by the middlesections. Also, there is much more difficulty in adhering them to thecovers or chassis as there are no continuous surfaces to apply a contactadhesive. This lack of continuous contact surfaces also causes the wiremesh to be of less value in term of radio frequency (RF) attenuation orelectrostatic discharge (ESD) conductivity.

[0006] Chassis designers face another general problem concerning gapclosure; non-uniform distortion of covers. Parts deflections under load(aside from coil springs) produce various complex deflection curves.This deflection curve, all too often, causes covers to bow away from thechassis to the point where a gap develops along the seam. Even aminiscule gap of a few thousandths of an inch ca cause the computer tofail EMI or ESD requirements.

[0007] An additional problem encountered is that a linear gasketprovides a force/unit length proportional to the compression in the sameunit length. In many cases, the compression is severely uneven over thelength that the gasket is being used. For example, on a hinged door witha latch on the outside edge, there would be much more compression (andmore force) toward the hinge and toward the latch than there would be inthe center of the door. Using a standard gasket tends to deform such adoor, and potentially does not provide enough force to electrically sealthe door in the center. What is ideally needed is a gasket that providesa varying force-compression curve along its length. Again, in the caseof a latched door, it would provide more force in the center, and lesstoward the hinge and latch, optimally providing a constant force perunit length while the door is closed and latched.

[0008] Therefore, what is needed is a gasket that provides EMI shieldingand generates less force than a traditional gasket, and that has theability to vary the force provided along the length of the gasket.

SUMMARY

[0009] One embodiment, accordingly, provides an EMI shielding gasketwhich reduces the closure force between the chassis closure surfaces andprovides enhanced EMI shielding. To this end, a gasket includes acompressible strip of EMI limiting material. A pattern of apertures isformed in the strip.

[0010] A principal advantage of this embodiment is that a moreconsistent linear sealing force is provided along the seam between thechassis closure surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 a diagrammatic view illustrating an embodiment of acomputer system.

[0012]FIG. 2 is perspective view illustrating an embodiment of a chassisin an open position.

[0013]FIG. 3 is a perspective view illustrating the chassis in a closedposition.

[0014]FIG. 4 is another perspective view illustrating the chassis in theopen position.

[0015]FIG. 5 is a further perspective view illustrating the chassis inthe closed position.

[0016]FIG. 6 is another perspective view illustrating the chassis in theopen position.

[0017]FIG. 7 is a perspective view illustrating a sealing gasket in atongue and groove engagement of a portion of the chassis.

[0018]FIG. 8 is a view illustrating a gasket including a plurality ofequidistantly spaced holes.

[0019]FIG. 9 is a view illustrating a gasket including a plurality ofvariably spaced holes.

[0020]FIG. 10 is a view illustrating a gasket including a plurality ofvariably sized holes.

[0021]FIG. 11 is a partial view illustrating a gasket having a roundhole.

[0022]FIG. 12 is a partial view illustrating a gasket having arectangular hole.

[0023]FIG. 13 is a partial view illustrating a gasket having a hexagonalhole.

[0024]FIG. 14 is a perspective view illustrating a chassis utilizing asealing gasket as disclosed herein.

[0025]FIG. 15 is a graphical view comparing gasket compression curves.

[0026]FIG. 16 is a graphical view comparing gasket compression curves.

DETAILED DESCRIPTION

[0027] In one embodiment, computer system 10, FIG. 1, includes amicroprocessor 12, which is connected to a bus 14. Bus 14 serves as aconnection between microprocessor 12 and other components of computersystem 10. An input device 16 is coupled to microprocessor 12 to provideinput to microprocessor 12.

[0028] Examples of input devices include keyboards, touchscreens, andpointing devices such as mouses, trackballs and trackpads. Programs anddata are stored on a mass storage device 18, which is coupled tomicroprocessor 12. Mass storage devices include such devices as harddisks, optical disks, magneto-optical drives, floppy drives and thelike. Computer system 10 further includes a display 20, which is coupledto microprocessor 12 by a video controller 22. A system memory 24 iscoupled to microprocessor 12 to provide the microprocessor with faststorage to facilitate execution of computer programs by microprocessor12. It should be understood that other busses and intermediate circuitscan be deployed between the components described above andmicroprocessor 12 to facilitate interconnection between the componentsand the microprocessor.

[0029] A chassis 26, FIG. 2, is provided to support all or most of thecomponents of system 10, as set forth above. Chassis 26 includes a baseportion 28 formed of a metal portion 30 and a cosmetic cover 32. A topportion 34 of chassis 26 is pivotally connected to base portion 28 at ahinge connection generally designated 36. Top portion 34 includes ametal portion 38 and a cosmetic cover 40. The base portion 28 includes abase surface 42. The cosmetic cover 40 includes a top surface 46 and anendwall 48. The base portion 28 forms part of a cavity 50 in chassis 26for containing a plurality of first computer components 52, and the topportion 34 forms another part of the cavity 50 for containing aplurality of second computer components 54.

[0030] The hinge connection 36 permits the top portion 34 to pivot to anopen position 0 about 90° relative to base portion 28, and to pivot to aclosed position C, FIG. 3, wherein the top portion 34 and base portionnest together to define the cavity 50. It is understood that the openposition 0 may be more or less than 90° as desired.

[0031] A pair of side panels 72, FIGS. 3 and 4, of top cosmetic cover 40are configured to nest with a complimentary configured pair of sidepanels 74 of base cosmetic cover 32 when chassis 26 is in the closedposition C. When closed, the top portion 34 is automatically secured tothe base portion 28 by a releasable latch 56, extending from each sidepanel 72 of top portion 34, which includes a latch member 56 a and arelease button 56 b which permits latch member 56 a to disengage frombase portion 28.

[0032] Pivotal movement of top portion 34, FIG. 2, relative to baseportion 28 is assisted by the hinge connection 36 including a pair ofarcuate guides 58 attached to base portion 28. A groove 60 in guides 58receives a pin 62 attached to top portion 34 for sliding movement inguides 58.

[0033] In FIG. 5, the metal chassis is illustrated including the metalbase portion 30 and the metal top portion 38. The hinge 36 is alsoillustrated including one of the arcuate guides 58, including groove 60,in the metal base portion 30, and one of the pins 62 attached to themetal top portion 38. This enables the top metal portion 38 is to pivotrelative to the base metal portion 30 between the open position 0 andthe closed position C, as described above.

[0034] The metal base portion 30 includes a pair of opposed basesidewalls 30 a, 30 b, FIGS. 5 and 6, and the metal top portion 38includes a pair of opposed top sidewalls 38 a, 38 b. The sidewalls 30 a,30 b, respectively matingly engage the sidewalls 38 a, 38 b. Preferably,the base sidewalls 30 a, 30 b include a tongue 31 and the top sidewalls38 a, 38 b include a groove 33, see also FIG. 7. A gasket 35 iscompressed into groove 33 so that a potentially harmful adhesive may notbe required to maintain the gasket 35 in place. Thus, when the tongue 31seats in groove 33, tongue 31 is sealingly engaged with gasket 35.Gasket 35 is preferably a fabric over foam EMI gasket sold under thename Foam Tite® by Advanced Performance Materials, Inc. (APM) of St.Louis, Mo.

[0035] In FIG. 8, gasket 35 includes a compressible strip of EMIlimiting material such as discussed above. FIGS. 8 and 9 respectivelyillustrate examples of rectangular and D-shaped gaskets. A pattern ofperforations such as holes 112 are formed through gasket 35.

[0036] The pitch P of holes 112, i.e. the center-to-center distancebetween adjacent holes 112 may be consistent or may vary along a lengthL of the gasket 35. FIG. 8 illustrates a consistent pitch P whereas,FIG. 9 illustrates a variable pitch P, P1, between the holes 112 to varythe compressibility of the gasket.

[0037] Also, FIG. 10 illustrates that compressibility can be varied byvarying the size of the holes 112 as is illustrated by a plurality ofholes 112 a, 112 b, each being of a different size such size S1 and S2,respectively.

[0038] In addition, the holes 112, FIGS. 11-13 can be of variablecross-sectional shapes. A hole 112 c, FIG. 11, is of a circularcross-section, a hole 112 d, FIG. 12, is of a rectangular cross-section,and a hole 112 e, FIG. 13, is of a hexagonal cross section. A rotary diecan be used to punch holes in gasket 35, as the gasket 35 is fed throughthe die.

[0039] The embodiments disclosed herein can be applied to any sort ofcontinuous cross-section (D-shaped, square, C-fold . . . etc.) gasketmaterial such as metalized fabric—foam core or conductive extrudedelastomers. In general, any shaped hole can be put into the gasket tomaximize the desired effect such as minimal forces or maximumconductivity, etc. Also, the pitch of hole the perforations can bevaried in order to match the deflection curve of the cover seams; aswell as, in combination with the above variations in hole pattern.

[0040] In FIG. 14, a chassis 120 includes a chassis body 122 and a pairof chassis covers 124 a, 124 b which are pivotally attached to body 122.Gaskets 35 may be selectively positioned along edge 126 of cover 124 afor engagement with edges 127 of chassis body 122. Also, additionalgaskets 35 are selectively positioned along edges 130 of cover 124 b forengagement with edges 131 of chassis body 122. In addition, gaskets 35(not viewable in FIG. 14) are positioned along edge 132 of cover 124 aand along edge 134 of cover 124 b, so that these gaskets 35 engage whencovers 124 a and 124 b are closed on the chassis body 122 such thatedges 132 and 134 overlap.

[0041]FIG. 15 illustrates the big improvement in force/length reductionfor a given gasket cross-section, when it is perforated according to thepresent disclosure. At point D the perforated gasket is 3 times softerthan the non-perforated one. The difference in conductivity at thispoint is only 4.4 milliohm-Ft. The gaskets represented here are aperforated and a non-perforated 74011 gasket from Chromerics.

[0042]FIG. 16 illustrates the comparison between the FIG. 15 perforatedgasket and a somewhat smaller/shorter gasket. Although they both share avery similar Conductivity Vs Compression curve, the comparison of ForceVs Compression shows that the smaller (non-perforated) gasket at point Bgenerates about 3.3 times as much force as the bigger/taller PerforatedGasket (which is 0.055″ taller than the shorter gasket!). The gasketsrepresented here are a perforated 74011 gasket from Chromerics and a4212 gasket from APM.

[0043] The reasons that the perforations do not adversely affect gasketperformance is threefold. Firstly, the perforations allow a much largersized (height/cross-section) gasket to be used for a given application(as stated above). Therefore the net contact area between cover andgasket may be substantially increased. Secondly, the conformability ofthe perforated gaskets are much better than their non-perforatedcounterparts along their length (as stated previously), and, in how wellthey flatten out. A regular non-perforated gasket will very oftenwrinkle or fold along it's periphery as it is compressed. This bothreduces the contact area between cover and chassis, and also increasesthe length of the conductive path going from cover to chassis. Thiswrinkling/folding effect increases the contact resistance and conductiveresistance of the gasket especially for rectangular cross sections. Infact, the primary (or only) reason there are D-shaped gaskets, versesrectangular, is in an attempt to produce softer more compliant gaskets.However, the D-shaped cross section generates only a small contact areain the lower range of compression (˜<30%), and the conductive path issignificantly longer as well. A rectangular gasket presents a largercontact throughout it's compression (and a shorter conductive path), butbecause of the high forces they generate, as well as the aforementionedproblems, the D-shaped gaskets are often (perhaps more often) used.However, when perforated, in accordance with these embodiments, therectangular cross sections are ideal for use in nearly all applications.Thirdly, because the type of gasket in these embodiments only conductsthru it's skin (metal plated fabric or metal foil) the contact area,along the centerline of the gasket, contributes little to the gasketsconductivity and hence can be removed without much impact, providedsufficient area is left to make conductive contact.

[0044] By removing large amounts of core material the gaskets are mademuch softer. These embodiments can be utilized on conductive elastomertype gaskets as well, and on various gasket cross sections. Thepreferred embodiment includes circular perforations with a ratio of openholes per gasket length of 0.687 (running along a centerline C of thegasket). The larger this ratio the softer the gasket. The above ratiotested to be good for ESD conductivity and EMI attenuation while vastlyreducing cover forces (approximately 3 times softer).

[0045] In the event that an adhesive is used, the perforations should beformed in the gaskets prior to laminating the PSA (pressure sensitiveadhesive) along the length of the gaskets. The perforations could beplaced by any number of means used in standard hole punching technology,however the preferred embodiment of the hole punching method would be touse a rotary die tool which would also have continuous rotary means forapplying the PSA after the hole punching.

[0046] As can be seen, the principal advantages of these embodiments arethat they reduce the closure force on a metal fabric/foil wrapped foamcore gasket by providing holes along the length of the gasket. The holegeometry can be varied to maximize effect. Additionally, the holegeometry can be varied along the length of the gasket to provide avariable force/compression curve to compensate for the geometry of theparts being closed by the gasket.

[0047] Additionally, the perforated gasket is much morecompliant/conformable along its length compared with its non-perforatedcounterpart. This is in terms of maintaining continuous contact surfacesalong its length over an obstacle in the chassis, or cover surfaces(screw heads, rivets, steps in sheet-metal lap joints, etc.)

[0048] The perforated gasket provides a generic form of EMI/ESD gasketwith the lowest forces possible, via perforations along its length (withimproved electrical/mechanical performance). That gasket also provides ameans of precisely controlling the force output of an EMI gasket viavarying pitch and/or size, and/or shape, of perforation holes along thelength of gasket.

[0049] Although illustrative embodiments have been shown and described,a wide range of modification, change and substitution is contemplated inthe foregoing disclosure and in some instances, some features of theembodiment may be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims-beconstrued broadly and in a manner consistent with the scope of theembodiments disclosed herein.

What is claimed is:
 1. A gasket comprising: a compressible strip ofelectromagnetic interference (EMI) limiting material; and a pattern ofapertures formed in the strip.
 2. The gasket as defined in claim 1wherein the apertures are equidistantly spaced apart.
 3. The gasket isdefined in claim 1 wherein the apertures are spaced apart by variabledistances.
 4. The gasket as defined in claim 1 wherein the apertures arevariably sized.
 5. The gasket as defined in claim 1 wherein theapertures have a circular cross-section.
 6. The gasket as defined inclaim 1 wherein the apertures have a rectangular cross-section.
 7. Thegasket as defined in claim 1 wherein the apertures have a hexagonalcross-section.
 8. A chassis comprising: a metal chassis first portion; ametal chassis second portion; a mating edge connection between the firstportion and the second portion; a gasket mounted in the edge connection,the gasket including a compressible strip of electromagneticinterference (EMI) limiting material; and a pattern of apertures formedin the strip.
 9. The gasket as defined in claim 8 wherein the aperturesare equidistantly spaced apart.
 10. The gasket as defined in claim 8wherein the apertures are variably spaced apart.
 11. The gasket asdefined in claim 8 wherein the apertures are variably sized.
 12. Thegasket as defined in claim 8 wherein the apertures have a circularcross-section.
 13. The gasket as defined in claim 8 wherein theapertures have a rectangular cross-section.
 14. The gasket as defined inclaim 8 wherein the apertures have a hexagonal cross-section.
 15. Acomputer system comprising: a chassis having an internal computercomponent cavity defined therein; a microprocessor mounted in thechassis; a storage coupled to the microprocessor; a video controllercoupled to the microprocessor; a memory coupled to provide storage tofacilitate execution of computer programs by the microprocessor; a firstportion of the chassis formed of a metal portion; a second portion ofthe chassis formed of a metal portion; a plurality of computercomponents mounted in the first portion of the metal chassis; a matingedge connection between the first and second portions of the metalchassis; and a gasket mounted in the edge connection, the gasketincluding a compressible strip of electromagnetic interference (EMI)limiting material; and a pattern of apertures formed in the strip. 16.The gasket as defined in claim 15 wherein the apertures areequidistantly spaced apart.
 17. The gasket is defined in claim 15wherein the apertures are variably spaced apart.
 18. The gasket asdefined in claim 15 wherein the apertures are variably sized.
 19. Thegasket as defined in claim 15 wherein the apertures have a circularcross-section.
 20. The gasket as defined in claim 15 wherein theapertures have a rectangular cross-section.
 21. The gasket as defined inclaim 15 wherein the apertures have a hexagonal cross-section.